A source of air pollution is the gasoline vapor in air vented from storage tanks for gasoline when those tanks are filled. Storage tanks, such as the underground storage tanks in filling stations and the fuel tanks in automobiles, draw air in as the volume of liquid is gradually depleted. Long contact with the liquid causes the air in the tank to become saturated with, or at least very enriched with, gasoline vapors. When liquid is added to the tank to replenish the supply, a volume of gasoline vapor-rich air essentially equal to the volume of liquid added is displaced and vented to the atmosphere unless some means for removing the gasoline vapor from the air is employed.